Assembly and Test of a Support Structure for 3.6 m Long Nb$_3$Sn Racetrack Coils

The LHC accelerator research program (LARP) is currently developing 4 m long Nb$_3$Sn quadrupole magnets for a possible upgrade of the LHC Interaction Regions (IR). In order to provide a reliable test bed for the fabrication and test of long Nb$_3$Sn coils, LARP has started the development of the long racetrack magnet LRS01. The magnet is composed of two 3.6 m long racetrack coils contained in a support structure based on an aluminum shell pre-tensioned with water-pressurized bladders and interference keys. For the phase-one test of the assembly procedure and loading operation, the structure was pre-stressed at room temperature and cooled down to 77 K with instrumented, solid aluminum "dummy coils". Mechanical behavior and stress homogeneity were monitored with strain gauges mounted on the shell and the dummy coils. The dummy coils were replaced with reacted and impregnated Nb$_3$Sn coils in a second assembly procedure, followed by cool-down to 4.5 K and powered magnet test. This paper reports on the assembly and loading procedures of the support structure as well as the comparison between strain gauge data and 3-D model predictions